Liquid level indicators are useful in a wide variety of applications and are known in a variety of different types. One type of level indicator which may be referred to as an analog indicator is capable of displaying the variable height of liquid in a container ranging, for example, between empty and full. This type of indicator is well known for use as a gasoline gage in automotive vehicles. Another commonly used type of liquid level indicator is that which signifies whether the level of liquid in a container is above or below a predetermined level. This type may be termed digital or "low" liquid level indicator.
The low liquid level indicator is useful in industrial machines, processing plants and in automotive vehicles for indicating whether or not a liquid level in a container is above or below a desired level. There is a need for an improved low liquid level indicator which will provide a reliable level indication under dynamic conditions. A dynamic liquid level indicator, as the term is used herein, means an indicator which is adapted to provide a level indication while there is relative movement between the liquid and its container such as splashing, sloshing, vibration and tilting. On the other hand, a static liquid level indicator, as the term is used herein, means one which is adapted to provide a level indication when there is no significant relative movement between the liquid and its container.
In certain applications, it is highly desirable to provide liquid level indication under dynamic conditions which result, for example, during operation of industrial machines or operation of an automotive vehicle. This is especially true where the machine or vehicle would be damaged or unsafe if operation is continued with a low level liquid such as liquid coolant, hydraulic fluid or liquid lubricant. In such cases, a static liquid level indicator is not adapted to provide a warning signal or indication in time to avoid operation under damaging or unsafe conditions.
In automotive vehicles, the liquid level in a container, such as the crankcase oil in the oil pan, will change at the location of a sensor probe with the tilting of the vehicle on a turn, the change of attitude of the vehicle on an upgrade and downgrade and in response to motion of the vehicle induced by bumps and the like. Unless special provision is made, a liquid level indicator will produce false indications under dynamic conditions. In the case of some liquid level indicator applications, a false indication may be induced by other operating conditions. For example, in the case of engine crankcase oil the oil level should be measured when the engine is warmed up so that there is consistently repeatable drain back of the oil from the engine passages into the oil pan. With the engine running and properly warmed up, a reliable indication of proper oil level can be obtained by appropriate placement of the level sensor probe in the oil pan. In this condition a better indication of oil level can be obtained than in the case of a cold engine where the amount of oil in the engine lubricating passages is variable or indeterminate.
The prior art includes many different forms liquid level indicators. The Ridpath et al U.S. Pat. No. 3,967,238 granted June 29, 1976 discloses a liquid level indicator for brake fluid in the master cylinders of a vehicle. A resistive probe is used in the liquid container and produces a signal which is proportional to the level of the liquid. A detector circuit includes a threshold detector to provide for switching at a discrete voltage level to produce a logic signal which controls the switching of an indicator lamp. An integrating network prevents transient voltage signals caused by sudden fluid level changes due to a change in attitude of the vehicle or splashing of the liquid.
The Dressler U.S. Pat. No. 4,356,480 granted Oct. 26, 1982 describes a liquid level indicator which provides indication of oil level during engine operation. This device. uses a conductive probe which produces a logic signal depending upon whether or not the probe is in contact with the oil. A latch circuit is provided with a clock input from a one-shot which clocks the oil level logic signal into the latch for storage. The one-shot has a clock input and when the ignition switch is closed it supplies a clock signal through a delay circuit to the clock input of the one-shot. The time delay is a small fraction of a second and allows the sensor signal to stabilize before the clock signal is applied to the latch. The one-shot is also provided with a time delay circuit which supplies a reset signal to the one-shot after a delay of thirty or forty seconds following the clock signal produced by closing the ignition switch. This time delay insures that the oil level reading, which is taken when the ignition switch is first turned on, is retained in the latch even though the engine does not start the first time and the ignition switch is turned on again within the time delay interval.
The Dressler U.S. Pat. No. 4,367,462 granted Jan. 4, 1983 and the Fiala U.S. Pat. No. 4,385,290 granted May 24, 1983 disclose liquid level sensing circuitry adapted for low oil level indication for a vehicle. In the systems of these two patents, an electronic circuit checks the oil level just after the engine is turned off and a logic signal is memorized to indicate whether the oil level is acceptable or unacceptable. Then, when the engine is later restarted, the oil level condition is indicated. This provides for measurement of the oil level after the engine is warmed up but it provides the indication to the driver upon restarting before the engine is warmed up. This system takes a reading of the oil level with the engine turned off and hence it is a static level indicator.
In the Kidd et al U.S. Pat. No. 4,503,419 granted Mar. 5, 1985, an oil level detection circuit detects oil level in the engine crankcase immediately after the ignition switch is turned on. An enabling circuit allows an output to the indicator only if the ignition switch has been turned off for a sufficient time period to allow the oil to drain back to the crankcase.
The Weiss U.S. Pat. No. 4,584,554 granted Apr. 22, 1986 discloses an engine oil level detecting device which uses a resistive probe to develop the voltage which varies according to the immersion of the probe in the oil. The voltage is applied to a galvanometer to indicate the oil level. A timer circuit maintains the signal to the galvanometer for a period of time after the ignition switch is closed so that the indication will not be changed by starting the engine and pumping of the oil.
A general object of this invention is to provide an improved dynamic liquid level indicator which overcomes certain disadvantages of the prior art.